python.pkgs.pytest: setupHook to prevent creation of .pytest-cache folder, fixes #40273

When `py.test` was run with a folder as argument, it would not only
search for tests in that folder, but also create a .pytest-cache folder.
Not only is this state we don't want, but it was also causing
collisions.

pkgs/development/python-modules/pytest/default.nix

@@ -1,6 +1,6 @@
 { stdenv, buildPythonPackage, fetchPypi, attrs, hypothesis, py
 , setuptools_scm, setuptools, six, pluggy, funcsigs, isPy3k, more-itertools
-, atomicwrites, mock
+, atomicwrites, mock, writeText
 }:
 buildPythonPackage rec {
   version = "3.6.2";
@@ -27,6 +27,11 @@ buildPythonPackage rec {
     runHook postCheck
   '';
 
+  # Don't create .pytest-cache when using py.test in a Nix build
+  setupHook = writeText "pytest-hook" ''
+    export PYTEST_ADDOPTS="-p no:cacheprovider"
+  '';
+
   meta = with stdenv.lib; {
     maintainers = with maintainers; [ domenkozar lovek323 madjar lsix ];
     platforms = platforms.unix;